The present invention generally relates to equipment and processes for depositing coatings. More particularly, this invention relates to coating equipment capable of depositing protective coatings within confined internal spaces of components exposed to high temperatures, such as the hostile thermal environment of a gas turbine, and to such components and their protective coatings.
Certain components of aircraft and industrial gas turbines, including gas turbine engines, steam turbines and wind turbines employed in power generation, require protective coatings for applications subjected to wear, corrosion, solid particle erosion, high temperatures, etc. Nonlimiting examples of protective coatings include metallic and ceramic-based coatings that provide wear, erosion, oxidation, corrosion, and/or thermal protection. Metallic coatings include diffusion coatings and overlay coatings, an example of the latter being MCrAlX coatings (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element). Overlay coatings are commonly deposited directly on the surface of a substrate by thermal spraying and/or electron beam physical vapor deposition (EBPVD). During subsequent exposures to high temperatures, such as during turbine operation, environmental coatings form a tightly adherent oxide layer, for example, alumina (Al2O3), that provides a barrier to oxidizing agents and other sources of environmental attack. A variety of ceramic materials are used to provide wear, corrosion, oxidation, erosion, and/or thermal protection. For wear, corrosion and erosion resistance, coating materials commonly used include WC—Co (typically about 5 to 50% Co by weight), TiN, TaC, Al2O3, TiO2, yttria-stabilized zirconia (YSZ), etc. To promote adhesion and extend the service life of a ceramic coating, an oxidation-resistant bond coat is often employed. Bond coats are typically in the form of a diffusion coating or overlay coating of the type noted above whose tightly adherent oxide (e.g., alumina) layer helps to adhere the ceramic coating to the bond coat.
Thermal spray deposition processes generally encompass such techniques as plasma spraying (air, vacuum and low pressure) and high velocity oxy-fuel (HVOF). Thermal spray processes involve propelling melted or at least heat-softened particles of a heat fusible material (e.g., metal, ceramic) against a surface, where the particles are quenched and bond to the surface to produce a coating. Coatings deposited by thermal spray processes are typically characterized by a degree of inhomogeneity and porosity that occurs as a result of the deposition process, in which “splats” of molten material are deposited and subsequently solidify. Due to the very high temperatures within the thermal spray, oxidation and/or phase changes of the deposited particles are common.
Cold spraying is a relatively new particulate deposition technique. As described in U.S. Pat. No. 5,302,414, cold spraying deposits a coating by propelling particles (powders) at high velocities, but at significantly lower temperatures compared to conventional thermal spray processes. A process gas (for example, helium, air, nitrogen, etc.) is used to accelerate the powder particles through a converging-diverging nozzle, yielding a supersonic gas flow and particle velocities of 300 m/s and higher. The process gas may be heated to a temperature of 800° C., but is more typically heated to less then 600° C. to minimize or eliminate in-flight oxidation and phase changes in the deposited material. As a result of the relatively low deposition temperatures and very high velocities, cold spray processes offer the potential for depositing well-adhering, dense, hard and wear-resistant coatings whose purity depends primarily on the purity of the powder used.
The gas flow in a converging-diverging nozzle operating in the choked condition is described by equation (1) below. Further details of gas flows in converging-diverging nozzles typically used in most cold spraying equipment can be understood from the theory of one-dimensional compressible fluid flow, published in various references including P. H. Oosthuizen and E. Carscallen, Compressible Fluid Flow (1997). Exit gas velocity will depend on the gun design, for example, the ratio of the area of the nozzle exit to the nozzle throat according to equation (1).A/A*=(1/M)[2/(γ+1)][1+((γ−1)/2)M2](γ+1)/2(γ−1)  (1)where A is the area at the nozzle exit, A* is the area of the nozzle throat, M is the Mach number of the exiting gas, and gamma (γ) is the adiabatic index or heat capacity ratio of the heat capacity at constant pressure (Cp) to the heat capacity at constant volume (Cv) for the process gas used. From equation (1), it is apparent that the gas flow parameters, including exit velocity, depend on the ratio A/A*. A certain minimum gas mass flow is necessary to operate the nozzle in the choked condition of equation (1) in order for the exit gas velocity to be the Mach number predicted by equation (1). Gases having higher gamma values are beneficial since they result in relatively higher Mach numbers. Increasing the gas mass flow rate beyond what is necessary to achieve the choked condition does not increase the gas exit velocity, but is advantageous for increasing the density of the gas in the nozzle. A denser gas is able to exert more drag force on the feedstock particles, and thus is more effective to accelerate the particles. In this manner, the effect of increasing the gas mass flow rate serves to increase the particle exit velocity. Higher particle velocities are generally preferred in cold spraying processes, since the particles must travel above a certain minimum critical velocity to form a well-adherent and dense coating in cold spraying. Higher gas mass flow rates for achieving higher particle exit velocities are often obtained by increasing the gas pressure at the gas inlet to the nozzle.
In contrast to gas mass flow rate and the geometry of the converging-diverging nozzle, the role of temperature in increasing the gas velocity in a cold spraying gun is somewhat indirect. As can be seen from the above equation (1), the gas temperature is not related to the exit Mach number. However, because the speed of sound increases with temperature, increasing gas temperature influences the exit gas velocity since a hotter gas will travel at an higher velocity.
Though offering the above advantages, the convergent-divergent design requirements of cold spray guns have resulted in relatively large guns, which has prevented the use of cold spraying processes for depositing coatings on surfaces within confined spaces and/or where access is limited or otherwise difficult. Particular examples include the interior surfaces of transition pieces of combustors for industrial gas turbines, where a dense metallic coating would be very beneficial for providing wear, corrosion and oxidation protection. As a result, interior surfaces of transition pieces are typically coated using air plasma spraying (APS), whose torches can be accommodated within the relatively small and confined interior of a transition piece. However, because APS processes are carried out at very high temperatures in an oxidizing atmosphere, traditional APS metallic coatings are porous and highly oxidized, which compromises the coating performance. According, it would be desirable to coat the interior surfaces of transition pieces and other hardware with a dense metallic coating that is substantially free of oxides and phase change effects, similar to the types of coatings that can be deposited by cold spraying.